I can not do one, but am working towards it. I'm currently working on pull ups and dips. I'm not sure how many pull ups I'm going to have to be able to do to get one. I can currently do 19 pullups and 35 dips. So it is the transition from one to the other that is my problem. Oh, and I weigh 240lbs. That doesn't help!!!

Most of those crossfit people do them with a kip. Large strength component is lost like that.
I seen a few people do them on dare, but most of those guys do one arm at a time. lol
Diesel, I thik you need to drill tansition, your pull ups and dips are up there. Mine are too, but lacking strength in transition.

Yes I can do them bar and on rings, bar I found was easier. If you can do 10 strict pullups with a one second pause at the bottom and top and dips you should be strong enough to do a muscle up.

To get stronger you can do pullups, getting bar above the sternum and start adding weights keeping the rep range around 5 or less. Focus on using your lats by pulling your elbows down and pretend you are trying the bend the bar as you go up. Always remember to keep the shoulders actively pulled down in the sockets so the ligaments do not take excessive load. With the rings you need to use a false grip, so you will need decent wrist strength but are healthier for your shoulders compared to the bar.